From WordNet (r) 3.0 (2006) [wn]:
roux
n 1: a mixture of fat and flour heated and used as a basis for
sauces
From The Collaborative International Dictionary of English v.0.48 [gcide]:
Roux \Roux\, n. [F. beurre roux brown butter.] (Cookery)
A thickening, made of flour, for soups and gravies.
[1913 Webster]